A sucker rod string is normally used in a pumped oil well to actuate the downhole pump. Such a string is made up of a multiplicity of sucker rods connected together end to end. Each such sucker rod comprises a solid steel cylindrical stem or rod, typically having a length of about 30 feet and a diameter of 5/8 or 3/4 inches. At each of its ends, the stem is provided with an upset or enlarged diameter portion having a threaded pin end.
A cylindrical internally threaded member, termed a "sucker rod coupling", is conventionally used to interconnect two sucker rods end to end. In this way, a "string" of sucker rods is built up or assembled.
The couplings are larger in outside diameter than the stem and the upset ends. Typically, a coupling has a diameter of 17/8".
The sucker rod string extends through and operates in a concentrically arranged tubing string. This tubing string is comprised of threaded-together steel tubes, typically each having a length of 30 feet and an inside diameter of 21/2 inches.
The rod string functions to actuate the downhole pump to force fluid to ground surface through the annular space defined between the rod and tubing strings. This actuation may occur as a result of reciprocating the rod string or, in the case where the downhole pump is of the screw type, rotating the rod string.
The present invention was developed in connection with rotating rod strings. It will be discussed herein in connection with the problems associated with such strings. However, the product can be used in reciprocating strings also.
The previously mentioned screw pumps are commonly used in connection with pumping heavy oil carrying a significant content of sand. Development of heavy oil reservoirs today commonly involves close spacing of the wells and drilling a number of the wells directionally from a single central "pad".
When the rod string is placed in such a well, it tends to assume a curvilinear configuration and it also tends to lie along the low side of the tubing. When the rod string is rotated to drive the pump, the longitudinally fixed but rotating couplings tend to slap and rub against the tubing wall. These actions, coupled with the erosive presence of sand trapped between the steel-on-steel coupling and tubing, can rapidly wear the tubing wall. In a "bad" well, where the well inclination, the longitudinally stationary positioning of the couplings, the relatively high speed rotation, and the presence of sand all combine to create an erosive environment, the rod string couplings can wear through the tubing wall in as little as two weeks. This result is disastrous for the well operation and requires expensive repairs.
It has long been conventional in the industry to centralize one string in another to keep them in spaced relation and out of contact. Commonly this has been done by mounting relatively large diameter annular bodies or `stabilizers` on the inner string at spaced points along its length.